1. Field of the Invention
The present invention relates to an image reading apparatus that converts light reflected from an original into analog image signals by a photoelectric conversion element, that converts the obtained analog image signals to which analog signal processing is applied into digital image signals by an analog-digital converter, and that outputs the digital image signals, and an image forming apparatus including the image reading apparatus.
2. Description of the Related Art
In recent years, reading speed of image data is increased, and the frequency of a reading operation clock is also increased accordingly. Consequently, it is essential to take a countermeasure against electromagnetic interference (EMI) relative to the regulations on electromagnetic wave intensity (such as Federal Communications Commission (FCC) and Voluntary Control Council for Interference by Information Technology Equipment (VCCI)).
A Spread Spectrum Clock Generator (SSCG) is generally used as an effective countermeasure against the EMI. The SSCG is a method for integrally planarizing the EMI spectrum by modulating the frequency of the operation clock within a very small area at a slow cycle rate.
When the SSCG is applied to an image reading apparatus, the adverse effect of modulation of the SSCG appears on an image. When the modulation of the SSCG is applied to a drive of a charge-coupled device (CCD) used as a photoelectric conversion element and an operation clock of analog-digital conversion (A/D conversion), the offset level of an image reading signal varies in synchronization with the modulation cycle of the SSCG, however slight. Accordingly, a streak PP such as a horizontal streak or an orthogonal streak appears on the image (see FIG. 16).
To solve such a problem, for example, Japanese Patent Application Laid-open No. 2008-118366 discloses a technology by which a variation component in an image signal is removed, and horizontal streaks are prevented from appearing.
Japanese Patent Application Laid-open No. 2008-118366 discloses a variation removing circuit 112 that is included in an image reading apparatus functioning to convert incident light into analog image signals by a photoelectric conversion element 105, to digitalize the analog image signals by an analog-digital converter, and to output the signals, and that drives the photoelectric conversion element 105 by a clock whose frequency is modulated, and superimposes a signal having an opposite phase and the same variation amount as those of the analog image signals on the image signal, corresponding to a frequency change of the clock (see FIG. 17).
The variation removing circuit 112 generates a correction signal whose amplitude and phase are set in advance to be superimposed on the image signal, and superimposes the correction signal on the image signal. As an example, it is possible to change the bias current of a transistor connected in an emitter follower configuration.
However, in such a conventional technology, the amplitude and the phase of the correction signal to be superimposed on the image signal are set in advance. Accordingly, when the offset variations due to the effect of the SSCG fluctuate among devices, the conventional technology may fail to deal with the situation.